Short-term object recognition memory in the rat: nonmatching with trial-unique junk stimuli

A delayed nonmatching-to-sample task with trial-unique stimuli (DNMS), similar to that used to test object recognition memory in primates, was adapted for use with rats. For each trial of the DNMS task, two stimuli were randomly selected from a pool of 250 small "junk" objects; one member of the pair was designated as the sample. On the first part of a trial, the rat traversed an elevated runway and displaced the sample for food reward. After a 10-s delay, the rat again traversed the runway to choose between the previously presented sample and the second member of the pair. Reward on the choice trial followed selection of the new object. Scores on the first day of DNMS were significantly above chance, and animals could consistently perform at approximately 75% accuracy. Extending the delay to 30 or 120 s lowered choice accuracy, but performance was still above chance. The DNMS taks for rats, unlike most other memory tests for rodents, does not require memory for spatial location. The similarity to tests used with primates should allow for more direct comparison of results of memory research across species.